good riffs

im currently writing a song , (metal with a shred solo) the solo is a 17-15-14------------17-15-14-------------------------15--14-12you know, kind of pattern, the song is mostly for the rhythm--------2---30---5pattern with a few different chords and hammers and pulloffs, and stuff you know. but im having trouble putting in a cool riff. i want one low register, but i really cant think of one at the moment. im kinda out of ideas, of the time, so if anyone has one please tell me, or suggest something. thank you very much.